1. Field of the Invention
The embodiments of the invention generally relate to integrated-optic communications, and more particularly to a multimode interference device for use in telecommunications, sensing, and related applications.
2. Description of the Related Art
Multimode interference (MMI) devices are useful for power splitting and for the separation/combination of wavelengths or polarizations (SCWP), usually in integrated-optic (IO) systems. A typical MMI device consists of a MMI region separated by an input port and an output port. The ports are where input/output guides connect to the MMI region. Typically, in conventional MMI devices, the input and output guides only connect to the ends of the MMI region; i.e., they are end-ported.
Power splitting devices are required for many signal distribution and sensing applications. As another example, wavelength separation/combination devices for 1.31 μm and 1.55 μm are required in the field of fiber optic communications to take advantage of embedded fiber optic systems at 1.31 μm and proceed with the deployment of the lower loss 1.55 μm systems. Wavelength separation/combination devices are also required for optical pumping of a 1.55 μm signal with a 0.98 μm pump. Commercially available devices typically rely on bulk optical filters. In another example, polarization separation/combination devices are required for various IO and fiber optic sensing systems, such as optical gyroscopes and structural stress sensors. Basically, the self-imaging effect is a general property of light. Thus, MMI devices do not have to be implemented in IO systems.
FIG. 1 illustrates a schematic top-down view of a conventional MMI-based separation/combination of wavelength/polarization (SCWP) device 1 consisting of a straight input guide 2 (which may be the output of a previous device) connected to an end-ported MMI region 3, which is connected to (1) a first straight end-ported output guide 4 for a first polarization or wavelength and (2) a second straight end-ported output guide 5 for a second polarization or wavelength.
The guides 2, 4, 5 and MMI region 3 may be defined using a variety of methods that are well-established in the prior art. The entire device 1 is nearly always single mode in the transverse (out of the paper) direction. The input/output guides 2, 4, 5 are commonly, but not always, single mode in the lateral (horizontal) direction also. Often the input/output guides 2, 4, 5 are expanded as they near the MMI region 3 because it has been shown that wider ports improve device performance. However, the expansion is typically performed adiabatically (i.e., in a gradual way) so that only the lowest order mode is being propagated. The MMI region 3, in contrast, supports many lateral modes. An input signal excites these modes, which propagate with different phase velocities down the length of the MMI region 3, and become de-phased. A self-image of the input to the MMI region 3 forms when the superposition of the modes in the image plane again matches the original modal distribution at the input plane. This condition occurs at planes where the accumulated phase differences among the excited modes are integral multiples of 2π, which allows the excited modes to constructively interfere and reproduce the input's modal distribution. The propagation distance at which this occurs is known as the self-image length.
The self-image length depends upon the polarization and the wavelength. A useful property of the self-imaging effect is that a lateral displacement of the input to the MMI region 3 along the object plane results in a corresponding displacement of the self-image along the image plane. This self-image displacement is antisymmetrical for the first self-image length (crossed). The second self-image is antisymmetrical to the first self-image, and thus symmetrically displaced relative to the input (barred). This continues, with odd-numbered self-images being antisymmetrically displaced (crossed), and even-numbered self-images being symmetrically displaced (barred). It has been previously shown that the dimensions of the MMI region 3 can be set such that the modes constructively interfere at the end of the region, forming a self-image of the transverse electric (TE) (or λ1) input signal at one output port and transverse magnetic (TM) (or λ2) at the other output port. Thus, the device 1 can separate two polarizations (or wavelengths) from one input guide 2 into two output guides 4, 5, each containing a different polarization (or wavelength). The device 1 can also work in reverse as a combiner.
FIG. 2 illustrates a schematic top-down view of a conventional MMI-based 1×2 power splitting device 11 consisting of a straight input guide 12 (which may be the output of a previous device) connected to an end-ported MMI region 13, which is connected to a first pair of straight output guides 14, 15, which connects to two first curving guides 16 respectively connecting to two second curving guides 17. A second pair of straight output guides 18, 19 (which may serve as inputs for subsequent devices) is also included connected to the second curving guides 17.
This device 11 uses the property of the self-imaging effect that multiple self-images of the input are formed at proper integer fractions of the self-imaging length. For an MMI region 13 with its length set to half of the self-image length, the output plane contains two self-images of the input. Each of the two output guides 14, 15 contains half of the power of the input guide 12 (ignoring the slight losses that occur). Typically, power splitters are combined with S-bends (i.e., first curving guides 16 and second curving guides 17) on the output guides 14, 15 as shown in FIG. 2, in order to laterally separate the outputs. The device 11 can also work in reverse as a power combiner, provided that the two input signals are coherent. For maximum recombination, the two input signals should be in phase.
Power splitters/combiners are used in a variety of applications. Many sensing and signal control architectures use a Mach-Zehnder Interferometer (MZI), or variations thereof. FIG. 3 schematically shows a conventional MZI 21, which consists of a straight input guide 22 operatively connected to an end-ported MMI 1×2 power splitter 23, which connects to four S-bend guides 27 (with individual parts not shown). A first straight guide 24 serving as a reference leg and a second straight guide 25 serving as a sensing leg are operatively connected to the MMI 1×2 power splitter 23. Connected to the second straight guide (sensing leg) 25 is a phase-changing mechanism 26 that changes the phase of the signal in the second straight guide (sensing leg) 25. An end-ported MMI 1×2 power combiner 28 is also included located at an opposite end from the MMI 1×2 power splitter 23 in the MZI 21. Two of the S-bend guides 27 serve as input guides 20, 30 into the MMI 1×2 power combiner 28, which then terminates with a straight output guide 29 extending out of the end of the MMI 1×2 power combiner 28. The output power from the power combiner varies as the phase of the signal of one port changes relative to the signal of the other port.
For signal distribution applications, 1×2 power splitters are often used in series to create 1×2 splitting, then 1×4 splitting, then 1×8 splitting, then 1×16 splitting, and so on. This is sometimes referred to as cascading 1×2 splitters. Such a structure is referred to as a 1×N power splitter. The schematic for such a structure is shown in FIG. 4, which is depicted as an end-ported MMI-based 1×16 power splitter 31 consisting of a straight input guide 32 connecting to one of fifteen end-ported MMI 1×2 power splitters 33. The MMI 1×2 power splitters 33 are configured in four rows of 1, 2, 4, and 8 power splitters in the respective rows. A row of two intermediate S-bend guides 34 (with the individual parts not shown) connects the first row of MMI 1×2 power splitters 33 to the second row of 1×2 power splitters 33. A row of four intermediate S-bend guides 35 (with the individual parts not shown) connects the second row of MMI 1×2 power splitters 33 to the third row of 1×2 power splitters 33. A row of eight intermediate S-bend guides 36 (with the individual parts not shown) connects the third row of MMI 1×2 power splitters 33 to the fourth row of 1×2 power splitters 33. Additionally, a row of sixteen output S-bend guides 37 (with the individual parts not shown) also connects to the fourth row of MMI 1×2 power splitters 33.
The conventional MMI-based power splitting and SCWP devices that have been previously described generally outperform competing conventional techniques. Like competing conventional techniques, however, their input/output ports are initially close enough in the lateral direction to allow the unwanted transfer of an input signal laterally from one guide to the other, as the signal is guided toward or away from the MMI region. Also, laterally separating the guides requires bends, which cause unwanted signal loss, both from mode mismatch at guide boundaries and from radiative effects of curvature. For 1×N power splitters, the bends in the guides that are necessary to separate the intermediate outputs often cause unwanted nonuniformity in the powers at the final outputs.
FIG. 5 shows a schematic top view of a conventional end-ported slotted MMI-based switch 41 consisting of a straight input guide 42 connecting to an end-ported MMI region 43, which then connects to (1) a first straight output guide 44 for the “slot off” condition and (2) a second straight output guide 45 for the “slot on” condition. Furthermore, a slot 46 is configured in the MMI region 43, wherein the slot 46 can be turned on and off in the following manner. In the “slot off” condition, light exits the MMI region 43 on the opposite lateral side 44 from the input guide 42. In the “slot on” condition, light exits the MMI region 43 on the same lateral side (i.e., through the output 45) as the input guide 42.
Unfortunately, there tends to be three weaknesses to this conventional design. First, the outputs 44, 45 must be subsequently separated by S-bends, which are not shown. Second, the slot width is constrained by the presence of the input/output guides 42, 44, 45 along the ends of the MMI region 43. Making the slot too wide, so that it overlaps with the input/output ports, lessens the throughput and increases the crosstalk, both of which are undesirable. Third, the ideal device length is slightly different for the “slot on” and “slot off” conditions. The length must be set to a compromise length, which compromises the overall performance of the switch 41.
Additionally, angling the input/output guides of conventional MMI devices has been previously suggested, but only in the context of end-porting. For end-porting, angled input/output guides degrade the performance unless the MMI region is non-rectangular (e.g., bow-tie shaped), which introduces additional difficulties in design and fabrication. Thus, there remains a need to improve the performance of the conventional MMI devices by increasing the lateral separation of the ports and by separating the guides without using bends.